Process for preparation of catalyst carrier and its use in catalyst preparation

ABSTRACT

This invention relates to catalyst carriers to be used as supports for metal and metal oxide catalyst components of use in a variety of chemical reactions. More specifically, the invention provides a process of formulating an alpha alumina carrier that is suitable as a support for silver and the use of such catalyst in chemical reactions, especially the epoxidation of ethylene to ethylene oxide. The composition comprises at least one hydrated precursor of alpha alumina; an optional alpha alumina; and a binder. The composition is substantially free of seeding particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to catalyst carriers to be used as supports formetal and metal oxide catalyst components which are then used in avariety of chemical reactions. More specifically, the invention pertainsto a process of formulating catalyst having a low surface area alphaalumina carrier that is suitable as a support for silver, and the use ofsuch catalyst in chemical reactions, especially the epoxidation ofethylene to ethylene oxide.

2. Description of the Related Art

Alumina is well known to be useful as a catalyst support for theepoxidation of olefins. It is particularly useful as a support for acatalyst comprising silver which is employed in the oxidation ofethylene to ethylene oxide. Support materials are made by fusing highpurity aluminum oxide, with or without silica. For this purpose thesupport material often comprises 90 percent or more, by weight, alphaalumina and up to 6 percent, by weight, silica. They are usually veryporous and have a high or low surface area depending on the use to bemade of them.

In known processes of making a support, alpha alumina and/or transitionalumina (alpha alumina precursors) is thoroughly mixed with temporaryand permanent binders. The temporary binders hold together thecomponents of the carrier precursor during its processing. The permanentbinders are inorganic materials having fusion temperatures below that ofthe alumina and induce fusion at the points of contact of the aluminaparticles, and impart mechanical strength to the finished support. Afterthorough dry-mixing, sufficient water is added to the mass to form themass into a paste-like extrudable admixture. The catalyst supportparticles are then formed from the paste by conventional means such ashigh pressure extrusion, tableting, granulation or other ceramic formingprocesses. The particles are then dried and are subsequently fired at anelevated temperature.

In the firing step, the temporary binders are burnt or thermallydecomposed to carbon dioxide and water, and are volatilized. It is knownin the art that ceramic carrier based catalysts comprise inert, solidsupports such as alpha alumina. Such have been described in U.S. Pat.Nos. 3,664,970; 3,804,781; 4,428,863 and 4,874,739. U.S. patents whichdescribe the making of alumina supports include U.S. Pat. Nos.2,499,675; 2,950,169 and 3,172,866. Such carriers have potentialapplications in the catalytic field especially where the alumina base isalpha alumina. Other patents such as U.S. Pat. Nos. 3,222,129; 3,223,483and 3,226,191 show the preparation of active aluminas. Methods of makinghighly porous aluminas are disclosed in U.S. Pat. Nos. 3,804,781;3,856,708; 3,907,512 and 3,907,982. Alumina carriers having high thermalstability are disclosed in U.S. Pat. No. 3,928,236. Other methods ofmaking catalyst carriers are discussed in U.S. Pat. Nos. 3,987,155;3,997,476; 4,001,144; 4,022,715; 4,039,481; 4,098,874 and 4,242,233.U.S. Pat. No. 3,664,970 discloses a carrier containing mainly aluminaand also contains silica, magnesia and titania. U.S. Pat. No. 4,410,453discloses that the performance of a silver on alumina catalyst for theoxidation of ethylene to ethylene oxide is improved by the inclusion ofan oxide, or oxide precursor, of zinc, lanthanum, or magnesium. U.S.Pat. No. 4,200,552 discloses a carrier that is made of α-alumina and atleast one of the compounds SiO₂, TiO₂, ZrO₂, CaO, MgO, B₂O₃, MnO₂, orCr₂O₃, as a sintering agent. U.S. Pat. No. 4,455,392 discloses thecomposition of an alumina carrier that contains silica and magnesia ascomponents of the bonding material. U.S. Pat. No. 5,100,859 discloses acarrier that contains an alkaline earth metal silicate, which may beadded as an original component or generated in situ by the reaction ofsilica, or silica generating compounds, with compounds that decompose toalkaline earth metal oxide upon heating. U.S. Pat. No. 5,512,530discloses a process for the production of a catalyst carrier which isbased on mixing alpha alumina, burnout material, and titania. U.S. Pat.No. 5,380,697 discloses a carrier containing a ceramic bond comprises60% wt. silica, 29% wt. alumina, 3% wt. calcium oxide, 2% magnesia, 4%wt. alkali metal oxides and less than 1% wt. each of ferric oxide andtitania. U.S. Pat. No. 5,733,840 and U.S. Pat. No. 5,929,259 disclose atitania-modification of formed carriers. The treatment involvedimpregnating the pre-formed carrier with a solution of titanyl oxalate,titanium (IV) bis(ammonium lactato)dihydroxide, or similar organic saltsand then the impregnated carrier is calcined at a temperature from about450 to 700° C. The patents disclosed that if titania is added during thecarrier's preparation, it tend to affect the densification of thecarrier structure which can lead to unacceptable properties. U.S. Pat.No. 4,368,144 states that better catalytic performance was obtained withcarriers that contain no more than 0.07% Na. U.S. Pat. No. 6,103,916patent discloses that catalyst performance was improved when the carrierwas washed by boiling in pure water until the water resistivity is morethan 10,000 Ω·cm. U.S. Pat. No. 5,384,302 patent disclosed an α-aluminabased carrier which prepared by mixing at least two alumina components.The first provides 95-40% of the total aluminum components and is madeof α-alumina of crystallite size of 0.4-4μ.

One of the problems with the catalysts that are based on porous carriersis that they have an insufficiently uniform pore structure. U.S. Pat.No. 4,022,715 attempts to solve this problem by using an organicsolution of a blowing agent, mixed with a carrier precursor composition.It has now been found that an improved carrier pore structure can beformed by employing a composition for a catalyst support which comprisesan admixture of a transition alumina alone with or without an alphaalumina having a median particle size of about 5μ or greater; and abinder. The composition is substantially free of seeding particles.Optionally the composition may have either a solid blowing agent whichexpands, or propels a gas upon the application of sufficient heat, talcand/or a water soluble titanium compound.

The catalyst support of this invention has excellent crush strength,porosity, and surface area on which the catalytic component will bedeposited. The optimum porosity and surface area are important forcatalytic function and insures the absence of diffusional resistancesfor reactants and product gases under reaction conditions. A minimumsurface area is important because it provides the structure on which thecatalytic component will be deposited. Crush strength is a parameter ofthe physical integrity of the carrier. This physical strength isessential for the catalyst ability to withstand handling as well as itslong life in a commercial reactor. A carrier that has the optimumsurface area and porosity may be deficient in its crush strength, andvice versa. The balance between the different physical specifications ofthe carrier is important. Normally, a carrier that has the optimumsurface area and porosity may be deficient in its crush strength, andvisa-versa. The proper choice of the specifications of the alphaalumina, that is used in the carrier composite, will help in the balancebetween the properties of the finished carrier. An optimum balancebetween these specifications is obtained via the choice of the carriercomponents according to the invention.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a composition for producing acatalyst support which comprises an admixture of at least one alphaalumina having a median particle size of about 5μ or greater, in anamount of from about 20 percent to about 60 percent by weight of totalalumina in the composition; at least one hydrated precursor of alphaalumina in an amount of from about 40 percent to about 80 percent byweight of total alumina in the composition; and a binder; whichcomposition is substantially free of seeding particles.

Another embodiment of the invention provides a process for producing acatalyst support which comprises:

-   a) preparing composition for producing a catalyst support which    comprises an admixture of at least one alpha alumina having a median    particle size of about 5μ or greater, in an amount of from about 20    percent to about 60 percent by weight of total alumina in the    composition; at least one hydrated precursor of alpha alumina in an    amount of from about 40 percent to about 80 percent by weight of    total alumina in the composition; and a binder; which composition is    substantially free of seeding particles; thereafter-   b) molding the resultant composition into a structure; thereafter-   c) heating said structure for a sufficient time and at a sufficient    temperature to form a porous structure, and thereafter-   d) heating the porous structure for a sufficient time and at a    sufficient temperature to convert transition alumina, to alpha    alumina, and form a porous alpha alumina structure, and to fuse the    porous structure and thereby form a catalyst support.

Another embodiment of the invention provides a process for producing acatalyst support which comprises:

-   a) preparing composition for producing a catalyst support which    comprises at least one hydrated precursor of alpha alumina; and a    binder; which composition is substantially free of alpha alumina,    and substantially free of seeding particles; thereafter-   b) molding the resultant composition into a structure; thereafter-   c) heating said structure for a sufficient time and at a sufficient    temperature to form a porous structure, and thereafter-   d) heating the porous structure for a sufficient time and at a    sufficient temperature to convert the hydrated precursor of alpha    alumina, to alpha alumina, and form a porous alpha alumina    structure, and to fuse the porous structure and thereby form a    catalyst support.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the invention provides a composition for producinga catalyst support which comprises an admixture of at least one alphaalumina having a median particle size of about 5μ or greater, in anamount of from about 20 percent to about 60 percent by weight of totalalumina in the composition; at least one hydrated precursor of alphaalumina in an amount of from about 40 percent to about 80 percent byweight of total alumina in the composition; and a binder; whichcomposition is substantially free of seeding particles. In thisembodiment, the alpha alumina is preferably present in an amount of fromabout 40 percent to about 60 percent by weight of total alumina in thecomposition. In this embodiment, the at least one hydrated precursor ofalpha alumina in an amount of from about 40 percent to about 60 percentby weight of total alumina in the composition. In the context of thisinvention, seeding particles are those which produce nucleation sitesfor the formation of alpha alumina from the hydrated precursor.

In another embodiment of the invention, the composition for producing acatalyst support which comprises at least one hydrated precursor ofalpha alumina; and a binder; which composition is substantially free ofalpha alumina, and substantially free of seeding particles.

The hydrated precursor of alpha alumina may comprise an aluminumhydroxide such as gibbsite, boehmite, diaspore, bayerite andcombinations thereof. The total amount of alumina, that is alpha aluminaplus hydrated precursor of alpha alumina (transition alumina) may bepresent in the composition in an amount of from about 80 weight % toabout 100 weight % based on the weight of the finished carrier. It ispreferably present in an amount of from about 90 weight % to about 99weight % based on the weight of the finished carrier, more preferablyfrom about 97 weight % to about 99 weight percent based on the weight ofthe finished carrier. Preferably the composition is substantially freeof ferric oxide, chromium oxide, and sub-micron size particles of alphaalumina.

The composition is prepared by forming a physical admixture of thecomposition components. The binder may be a temporary binder, apermanent binder, or both. The temporary binders are thermallydecomposable organic compounds of moderate to high molecular weight. Thepermanent binders are inorganic clay-type materials that impartmechanical strength to the finished support.

Temporary binders, and burnout materials, include thermally decomposableorganic compounds, polyolefin oxides, oil, e.g. mineral oil, acacia,carbonaceous materials such as coke, carbon powders, graphite,cellulose, substituted celluloses, e.g. methylcellulose, ethylcellulose,and carboxyethylcellulose, cellulose ethers, stearates, such as stearateesters, e.g. methyl or ethyl stearate, waxes, powdered plastics such aspolyolefins, particularly polyethylene and polypropylene, polystyrene,polycarbonate, sawdust, starch, and ground nut shell flours, e.g. pecan,cashew, walnut and filbert shells, and combinations thereof, and thelike which burn at the firing temperatures employed. Burnout material isused primarily to ensure the preservation of the structure during thegreen, or unfired phase, in which the mixture may be shaped intoparticles by molding or extrusion processes and also provide the desiredporosity to the finished product. When employed, a temporary binder isessentially totally removed during the firing to produce the finishedsupport. The supports of the invention are preferably made with theinclusion of a permanent binder material to ensure preservation of theporous structure and to give added strength to the carrier after thecarrier is fired. Permanent binders, include inorganic clay materials,silica, alkaline earth metal oxides, alkali metal oxides, and titaniumoxide, silicates of elements of Group II of the Periodic Table of theelements, and combinations thereof. Useful clays non-exclusively includekaolinite. A convenient binder material which may be incorporated withthe alumina particles is a mixture of boehmite, stabilized silica soland a soluble sodium salt. Suitable bond materials for this inventioninclude calcium silicate and magnesium silicate either added as such orformed in situ. It is preferred, however, to use no free alkali metals,or their oxides. The binder may be present in the precursor in an amountof from about 0.1 weight % to about 15 weight % based on the weight ofthe composition, preferably from about 0.2 weight % to about 10 weight %based on the weight of the composition, and more preferably from about0.5 weight % to about 5 weight % based on the weight of the composition.

The composition may optionally contain a solid blowing agent whichexpands, or propels a gas upon the application of sufficient heat. Inone embodiment, the blowing agent comprises a composition ofmicrospheres which include thermoplastic shells which encapsulate ahydrocarbon. The hydrocarbon expands the thermoplastic shells upon theapplication of sufficient heat. Such blowing agents comprise gas-tightthermoplastic shells that may encapsulate a hydrocarbon in liquid form.Upon heating, the hydrocarbon is gasified and increases its pressurewhile the thermoplastic shell softens, resulting in an increase in thevolume of the microspheres. Examples of expandable microspheres areAdvancell, acrylonitrile-based spheres, commercially available fromSekisui Chemical Co. (Osaka, Japan) and Expancel® microspheres,commercially available from Expancel, Stockviksverken, Sweden. Expancelis available in unexpanded and expanded microsphere forms. Unexpandedmicrospheres have a diameter of from about 6 to about 40 μm, dependingon grade. When heated, these microspheres expand to from about 20 toabout 150 μm in diameter. The preferred hydrocarbon inside the shell isisobutane or isopentane. The shell is preferably a copolymer ofmonomers, e.g. vinylidene chloride, acrylonitrile and methylmethacrylate. In another embodiment, the blowing agent may be a solid,granular chemical blowing agent which decomposes upon heating, releasinga considerable amount of gaseous decomposition products and resulting inpore formation. Chemical blowing agents are preferably solid forms ofhydrazine derivatives that will release gases such as CO₂ and nitrogen.Examples of chemical blowing agents are p-toluenesulfonylhydrazide,benzenesulfonylhydrazide, and azodicarbonamide, H₂NCO—N═N—CONH₂.Azodicarbonamide decomposes at 200° C., into N₂, CO, and CO₂.

A suitable amount of blowing agent to provide the desired porosity maybe in the range of from about 0.1 weight % to about 30 weight % byweight of the overall composition. Preferably, the amount of the blowingagent ranges from about 1 weight % to about 20 weight % and morepreferably from about 3 weight % to about 15 weight percent based on theweight of the composition. The amount of blowing agent is a function ofits type, the type of alpha alumina and/or a transition aluminacomponents used, as well as the nature of the porosity that is desiredin the finished product.

After thorough dry-mixing of the alpha alumina, hydrated precursor ofalpha alumina material, binder and optional blowing agent, sufficientwater is added to the precursor mass to form a paste-like substance.Water and/or a water-containing substance is added to the startingprecursor in order to give plasticity to the mixture. The plasticmixture is then formed into the desired shape via standard ceramicprocessing methods, e.g. tableting or extrusion. The amount of wateradded to the carrier precursor will be a function of the method used toform the paste. Extrusion may require the addition of a higher level ofwater to gain the optimum level of plasticity. Where particles areformed by extrusion it may be desirable to include conventionalextrusion aids such as lubricants, for example petroleum jelly, ormineral oil. The lubricant may be present in the precursor in an amountof from about 0.1 weight % to about 10 weight percent based on theweight of the composition, preferably from about 0.5 weight % to about 5weight % based on the weight of the composition, and more preferablyfrom about 1 weight % to about 3 weight % based on the weight of thecomposition. The amounts of the components to be used are to some extentinterdependent and will depend on a number of factors that relate to theequipment used. However these matters are well within the generalknowledge of a person skilled in the art of extruding ceramic materials.Preparation of a catalyst carrier generally uses a step of kneading theprecursor material into a desired shape and a desired size. The catalystsupport particles are then formed from the paste by conventional meanssuch as, for example, pelletizing, high pressure extrusion, granulationor other ceramic forming processes. For use in commercial ethylene oxideproduction applications, the supports are desirably formed intoregularly shaped pellets, spheres, rings, particles, chunks, pieces,wagon wheels, cylinders, trilobes, tetralobes and the like of a sizesuitable for employment in fixed bed reactors. Desirably, the supportparticles may have “equivalent diameters” in the range of from about 3mm to about 20 mm and preferably in the range of from about 4 mm toabout 12 mm, which are usually compatible with the internal diameter ofthe tube reactors in which the catalyst is placed. “Equivalent diameter”is the diameter of a sphere having the same external surface (i.e.neglecting surface within the pores of the particle) to volume ratio asthe support particles being employed. The particles are then dried andare subsequently fired at an elevated temperature. The function of thedrying step is to remove the water from the shaped pellets. The formedcarrier precursor is dried to at a temperature of from about 80° C. toabout 150° C. for a time sufficient to remove substantially all of thewater. Then the extruded material is calcined under conditionssufficient to remove the burnout agents and the organic binding agentsand to fuse the alpha alumina particles into a porous, hard mass. Thecarrier is heated at a temperature that is high enough to sinter thealumina particles and produce a structure with physical propertiesadequate to withstand the environment in which it is expected tooperate. The calcination temperature and duration should be high enoughto convert any transition alumina into alpha alumina and to induce grainboundary fusion. Controlling the calcination process is essential toobtain a carrier having the optimum balance between surface area,porosity and strength. Typically the calcination temperature is higherthan 1000° C., preferably is in the range of 1150° C. to about 1600° C.The holding times at these maximum temperatures typically range fromabout 0 hour to 10 hours, preferably from about 0.1 hour to about 10hours preferably from about 0.2 hour to about 5 hours to form thesupport.

The final carrier has a water pore volume ranging from about 0.2 cc/g toabout 0.8 cc/g, preferably from about 0.25 cc/g to about 0.6 cc/g. TheBET surface area of the finished carrier is preferred to be in the rangeof 0.4-4.0 m²/g, more preferably from about 0.6 to about 1.5 m²/g. Thesuitable value of crush strength is about 8 pounds and higher,preferably about 10 pounds and higher, and more preferably about 14pounds and higher. Suitable porosity is expected to be in the range offrom about 20 to about 80%, preferably from about 25 to about 50%.

In another embodiment of the invention, the catalyst support is preparedas above except the composition comprises talc instead of or in additionto the permanent binder component. The talc may be present in theprecursor in an amount of from about 0.1 weight % to about 15 weight %based on the weight of the composition, preferably from about 0.5 weight% to about 10 weight % based on the weight of the composition, and morepreferably from about 1 weight % to about 8 weight % based on the weightof the composition. The support is then formed in a manner similar tothat described above.

In another embodiment of the invention, the support is prepared as aboveexcept the composition comprises a water soluble titanium compoundinstead of or in addition to the permanent binder. Suitable watersoluble titanium compounds non-exclusively include ammoniumhexafluorotitanate, titanyl oxalate, and titanium (IV) bis(ammoniumlactato)dihydroxide. The water soluble titanium compound may be presentin the precursor in an amount of from about 0.01 weight % to about 10weight percent based on the weight of the composition, preferably fromabout 0.1 weight % to about 8 weight percent based on the weight of thecomposition, and more preferably from about 0.2 weight % to about 5weight percent based on the weight of the composition. The support isthen formed in a manner similar to that described above. The optionaladdition of a small amount of boron, added as boric acid or a borate,also gives good results. The amount of boron added is in the range of0.0 to 3% based on the dry weight of the alumina used.

In order to produce a catalyst for the oxidation of ethylene to ethyleneoxide, a support formed above is then provided with a catalyticallyeffective amount of silver thereon. The catalysts are prepared byimpregnating the supports with silver ions, compounds, complexes and/orsalts dissolved in a suitable solvent sufficient to cause deposition ofsilver precursor compound onto the support. The impregnated carrier isthen removed from the solution and the deposited silver compound isreduced to metallic silver by high temperature calcination. Alsopreferably deposited on the support either prior to, coincidentallywith, or subsequent to the deposition of the silver are suitablepromoters in the form of ions, compounds and/or salts of an alkali metaldissolved in a suitable solvent. Also deposited on the carrier eitherprior to, coincidentally with, or subsequent to the deposition of thesilver and/or alkali metal, suitable transition metal compounds,complexes and/or salts dissolved in an appropriate solvent.

The supports as formed above are impregnated with a silver impregnatingsolution, preferably an aqueous silver solution. The support is alsoimpregnated at the same time or in a separate step with various catalystpromoters. Preferred catalysts prepared in accordance with thisinvention contain up to about 45% by weight of silver, expressed asmetal, deposited upon the surface and throughout the pores of a poroussupport. Silver contents, expressed as metal, of from about 1 to about40% based on weight of total catalyst are preferred, while silvercontents of from about 8 to about 35% are more preferred. The amount ofsilver deposited on the support or present on the support is that amountwhich is a catalytically effective amount of silver, i.e., an amountwhich economically catalyzes the reaction of ethylene and oxygen toproduce ethylene oxide. As used herein, the term “catalyticallyeffective amount of silver” refers to an amount of silver that providesa measurable conversion of ethylene and oxygen to ethylene oxide andselectivity and activity stability within catalyst life. Useful silvercontaining compounds non-exclusively include silver oxalate, silvernitrate, silver oxide, silver carbonate, a silver carboxylate, silvercitrate, silver phthalate, silver lactate, silver propionate, silverbutyrate and higher fatty acid salts and combinations thereof.

This catalyst comprises a catalytically effective amount of silver, apromoting amount of alkali metal, a promoting amount of a transitionmetal supported on a porous, support. As used herein the term “promotingamount” of a certain component of a catalyst refers to an amount of thatcomponent that works effectively to provide an improvement in one ormore of the catalytic properties of that catalyst when compared to acatalyst not containing said component. The exact concentrationsemployed, of course, will depend upon, among other factors, the desiredsilver content, the nature of the support, the viscosity of the liquid,and solubility of the silver compound.

In addition to silver, the catalyst also contains an alkali metalpromoter selected from lithium, sodium, potassium, rubidium, cesium orcombinations thereof, with, cesium being preferred. The amount of alkalimetal deposited on the support or catalyst or present on the support orcatalyst is to be a promoting amount. Preferably the amount will rangefrom about 10 ppm to about 3000 ppm, more preferably from about 50 ppmto about 2000 ppm and even more preferably from about 100 ppm to about1500 ppm and yet even more preferably from about 200 ppm to about 1000ppm by weight of the total catalyst, measured as the metal.

The catalyst also preferably contains a transition metal promoter whichcomprises an element from Groups 4b, 5b, 6b, 7b and 8 of the PeriodicTable of the Elements, and combinations thereof. Preferably thetransition metal comprises an element selected from Group 6b, and 7b ofthe Periodic Table of the Elements. More preferred transition metals arerhenium, molybdenum, and tungsten, with molybdenum and rhenium mostpreferred. The amount of transition metal promoter deposited on thesupport or catalyst or present on the support or catalyst is to be apromoting amount. The transition metal promoter may be present in anamount of from about 0.1 micromoles per gram to about 10 micromoles pergram, preferably from about 0.2 micromoles per gram to about 5micromoles per gram, and more preferably from about 0.5 micromoles pergram to about 4 micromoles per gram of total catalyst, expressed as themetal.

The silver solution used to impregnate the support may also comprise anoptional solvent or complexing/solubilizing agent, such as are known inthe art. A wide variety of solvents or complexing/solubilizing agentsmay be employed to solubilize silver to the desired concentration in theimpregnating medium. Useful complexing/solubilizing agents includeamines, ammonia, or lactic acid. Amines include alkylene diamines, andalkanol amines having from 1 to 5 carbon atoms. In one preferredembodiment, the solution comprises an aqueous solution of silver oxalateand ethylene diamine. The complexing/solubilizing agent may be presentin the impregnating solution in an amount of from about 0.1 to about 5.0moles of ethylene diamine per mole of silver, preferably from about 0.2to about 4.0 moles, and more preferably from about 0.3 to about 3.0moles of ethylene diamine for each mole of silver.

When a solvent is used it may be water-based, or organic-based, and maybe polar or substantially or totally non-polar. In general, the solventshould have sufficient solvating power to solubilize the solutioncomponents. At the same time, it is preferred that the solvent be chosento avoid having an undue influence on or interaction with the solvatedpromoters. The concentration of silver salt in the solution is in therange of from about 1% by weight to the maximum permitted by thesolubility of the particular salt/solubilizing agent combinationemployed. It is generally very suitable to employ silver salts solutionscontaining from 5% to about 45% by weight of silver with silver saltconcentrations of from 10 to 35% by weight being preferred.

Impregnation of the selected support is achieved in conventional mannersby excess solution impregnation, incipient wetness, etc. Typically thesupport material is placed in the silver solution until a sufficientamount of the solution is absorbed by the support. Preferably thequantity of the silver solution used to impregnate the porous support isno more than is necessary to fill the pore volume of the support. Thesilver containing liquid penetrates by absorption, capillary actionand/or vacuum into the pores of the support. A single impregnation or aseries of impregnations, with or without intermediate drying, may beused, depending in part on the concentration of the silver salt in thesolution. Impregnation procedures are described in U.S. Pat. Nos.4,761,394, 4,766,105, 4,908,343, 5,057,481, 5,187,140, 5,102,848,5,011,807, 5,099,041 and 5,407,888, which are incorporated herein byreference. Known prior procedures of pre-deposition, co-deposition andpost-deposition of various promoters can be employed.

Examples of catalytic properties include, inter alia, operability(resistance to runaway), selectivity, activity, conversion, stabilityand yield. It is understood by one skilled in the art that one or moreof the individual catalytic properties may be enhanced by the “promotingamount” while other catalytic properties may or may not be enhanced ormay even be diminished. It is further understood that differentcatalytic properties may be enhanced at different operating conditions.For example, a catalyst having enhanced selectivity at one set ofoperating conditions may be operated at a different set of conditionswherein the improvement shows up in the activity rather than theselectivity and an operator of an ethylene oxide plant willintentionally change the operating conditions in order to take advantageof certain catalytic properties even at the expense of other catalyticproperties in order to optimize conditions and results by taking intoaccount feedstock costs, energy costs, by-product removal costs and thelike. The particular combination of silver, support, alkali metalpromoter, and transition metal promoter of the instant invention willprovide an improvement in one or more catalytic properties over the samecombination of silver and support and none, or only one promoter.

After impregnation, the support impregnated with silver precursorcompound and the promoters is calcined or activated, for a timesufficient to reduce the silver component to metallic silver and toremove the solvent and volatile decomposition products from the silvercontaining support. The calcination is accomplished by heating theimpregnated support, preferably at a gradual rate, to a temperature inthe range of from about 200° C. to about 600° C., preferably from about230° C. to about 500° C., and more preferably from about 250° C. toabout 450° C., at a reaction pressures in the range of from 0.5 to 35bar, for a time sufficient to convert the contained silver to silvermetal and to decompose all or substantially all of present organicmaterials and remove the same as volatiles. In general, the higher thetemperature, the shorter the required calcination period. A wide rangeof heating periods have been suggested in the art to thermally treat theimpregnated support, e.g., U.S. Pat. No. 3,563,914 suggests heating forless than 300 seconds, U.S. Pat. No. 3,702,259 discloses heating from 2to 8 hours at a temperature of from 100° C. to 375° C. to reduce thesilver salt in the catalyst; usually for from about 0.5 to about 8hours, however, it is only important that the reduction time becorrelated with temperature such that substantially complete reductionof silver salt to catalytically active metal is accomplished. Acontinuous or step-wise heating program may be used for this purpose.

The impregnated support is maintained under an atmosphere comprising aninert gas and optionally an oxygen containing oxidizing component. Inone embodiment the oxidizing component is present in an amount of fromabout 10 ppm to about 5% by volume of gas. For purposes of thisinvention, inert gases are defined as those which do not substantiallyreact with the catalyst producing components under the catalystpreparation conditions chosen. These include nitrogen, argon, krypton,helium, and combinations thereof, with the preferred inert gas beingnitrogen. In a useful embodiment, the atmosphere comprises from about 10ppm to about 1% by volume of a gas of an oxygen containing oxidizingcomponent. In another useful embodiment, the atmosphere comprises fromabout 50 ppm to about 500 ppm of a gas of an oxygen containing oxidizingcomponent.

Ethylene Oxide Production

Generally, the commercially practiced ethylene oxide productionprocesses are carried out by continuously contacting an oxygencontaining gas with ethylene in the presence of the present catalysts ata temperature in the range of from about 180° C. to about 330° C. andpreferably about 200° C. to about 325° C., more preferably from about210° C. to about 270° C., at a pressure which may vary from aboutatmospheric pressure to about 30 atmospheres depending on the massvelocity and productivity desired. Higher pressures may, however, beemployed within the scope of the invention. Residence times inlarge-scale reactors are generally on the order of about 0.1-5 seconds.Oxygen may be supplied to the reaction in an oxygen containing stream,such as air or as commercial oxygen. The resulting ethylene oxide isseparated and recovered from the reaction products using conventionalmethods. However, for this invention, the ethylene oxide processenvisions the normal gas recycle encompassing carbon dioxide recycle inthe normal concentrations, e.g., about 0.1-15 volume percent. A usualprocess for the oxidation of ethylene to ethylene oxide comprises thevapor phase oxidation of ethylene with molecular oxygen in the presenceof the inventive catalyst in a fixed bed, tubular reactor. Conventionalcommercial fixed bed ethylene oxide reactors are typically in the formof a plurality of parallel elongated tubes (in a suitable shell)approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5 inches I.D. and15-45 feet long filled with catalyst.

The inventive catalysts have been shown to be particularly selectivecatalysts in the oxidation of ethylene with molecular oxygen to ethyleneoxide. The conditions for carrying out such an oxidation reaction in thepresence of the catalysts of the present invention broadly comprisethose described in the prior art. This applies, for example, to suitabletemperatures, pressures, residence times, diluent materials such asnitrogen, carbon dioxide, steam, argon, methane, the presence or absenceof moderating agents to control the catalytic action, for example, ethylchloride, 1,2-dichloroethane, or vinyl chloride, the desirability ofemploying recycle operations or applying successive conversion indifferent reactors to increase the yields of ethylene oxide, and anyother special conditions which may be selected in processes forpreparing ethylene oxide. Molecular oxygen employed as a reactant may beobtained from conventional sources. The suitable oxygen charge may berelatively pure oxygen, a concentrated oxygen stream comprising oxygenin major amount with lesser amounts of one or more diluents such asnitrogen, argon, etc., or another oxygen containing stream such as air.The use of the present catalysts in ethylene oxidation reactions is inno way limited to the use of specific conditions among those which areknown to be effective.

The resulting ethylene oxide is separated and recovered from thereaction products by conventional methods known and used in the art. Useof the silver catalysts of the invention in ethylene oxide productionprocesses gives higher overall ethylene oxidation selectivities toethylene oxide at a given ethylene conversion than are possible withconventional catalysts.

In the production of ethylene oxide, reactant feed mixtures may contain0.5 to 45% ethylene and 3 to 15% oxygen, with the balance comprisingcomparatively inert materials including such substances as nitrogen,carbon dioxide, methane, ethane, argon and the like. In a preferredapplication of the silver catalysts of the invention ethylene oxide isproduced when an oxygen containing gas of about 95% or more of oxygen.Only a portion of the ethylene usually is reacted per pass over thecatalyst and after separation of the desired ethylene oxide product andthe removal of appropriate purge stream and carbon dioxide to preventuncontrolled build up of inerts and/or by-products, unreacted materialsare returned to the oxidation reactor. For purposes of illustrationonly, the following are conditions that are often used in currentcommercial ethylene oxide reactor units. GHSV—1500-10,000; Inletpressure—150-400 psig; Inlet Feed: ethylene 1-40%; O₂—3-12%;CO₂—0.1-40%; ethane 0-3%; argon and/or methane and/or nitrogen argonand/or methane and/or nitrogen: 0.3-20 ppmv total diluentchlorohydrocarbon moderator; coolant temperature—180-315° C.; catalysttemperature 180° C.; O₂ conversion level—10-60%; EO Production (WorkRate) 2-16 lbs. EO/cu.ft. catalyst/hr.

The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1

The following components were mixed thoroughly:

100 g alpha alumina(I)* 120 g alpha alumina(II)** 290 g Hydrated alumina(Gibssite) 60 g Boehmite 14 g K15M Methocel (methyl cellulose) 2 gAcacia 94 g polycarbonate powder 25 g Mineral oil 8 g 50% Ti solution(as titanium bis-ammonium lactato dihydroxide) 30 Talc 143 g Water*Alpha Alumina(I) is a highly pure alpha alumina that has a BET surfacearea of 0.7 m²/g and Na contents of less than 0.3%. **Alpha Alumina(II)is a highly pure alpha alumina that has a BET surface area of 11 m²/gand Na contents of less than 0.15%.Step 1

All the dry components were mixed together, in a dry powder mixer (USStoneware Model M93120DC). The dry mixture was transferred to a highshear mixer (Lancaster Model 530PO). There it was blended with the waterand the water-soluble components and mixing continued for 15 moreminutes.

Step 2

The plastic mixture was extruded into 8 mm hollow cylinders, using aKillion extruder (Model 4321111282).

Step 3

The shaped pellets were dried at 120° C. and this was followed by firingin a slow and programmed scheme. The firing process involved heating thegreen ware in a high temperature furnace, using a CM furnace (Model1720). The firing scheme involved temperature ramping at a rate of 4°C./min up to 1275° C. The temperature of the furnace was held at thislevel for 2 hours and then it was allowed to cool down at a rate of 6°C./min. This carrier is designated as carrier A. Testing the carriershowed that it has the following specifications:

Testing the carrier showed that it has the following specifications:

Crush strength 19.2 lb Water absorption 32.3 ml/100 g BET surface area1.03 m²/g

EXAMPLE 2

The following components were mixed thoroughly:

176 g alpha alumina(II)** 232 g Hydrated alumina (Gibssite) 48 gBoehmite 15 g K15M Methocel (methyl cellulose) 2 g Acacia 80 gazodicarbonamide 20 g mineral oil 6.2 g 50% Ti solution (as titaniumbis-ammonium lactato dihydroxide) 16 g Talc 105 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1350° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier B. Testingthe carrier showed that it has the following specifications:

Crush strength 17.4 lb Water absorption 29.6 ml/100 g BET surface area1.03 m²/g

EXAMPLE 3

The following components were mixed thoroughly:

240 g alpha alumina(I)* 200 g alpha alumina(II)** 580 g Hydrated alumina(Gibssite) 120 g Boehmite 28 g A4 Methocel (methyl cellulose) 94 gPolycarbonate powder 50 g mineral oil 15.6 g 50% Ti solution (astitanium bis-ammonium lactato dihydroxide) 40 g Talc 216 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1350° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier C. Testingthe carrier showed that it has the following specifications:

Crush strength 19.4 lb Water absorption 31.7 ml/100 g BET surface area1.01 m²/g

EXAMPLE 4

The following components were mixed thoroughly:

176 g alpha alumina(II)** 232 g Hydrated alumina (Gibssite) 48 gBoehmite 12 g K15M Methocel (methyl cellulose) 30 g powdered walnutshells 20 g Mineral oil 6.2 g 50% Ti solution (as titanium bis-ammoniumlactato dihydroxide) 16 Talc 108 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1350° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. Testing the carrier showed that it has thefollowing specifications:

Crush strength 15.4 lb Water absorption 30.6 ml/100 g BET surface area0.93 m²/g

EXAMPLE 5

The following components were mixed thoroughly:

220 g alpha alumina(II)** 282 g Boehmite 15 g K15M Methocel (methylcellulose) 50 g Polycarbonate powder 25 g Mineral oil 7.8 50% Tisolution (as titanium bis-ammonium lactato dihydroxide) 20 Talc 108 gWater

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1350° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier D. Testingthe carrier showed that it has the following specifications:

Crush strength 14.5 lb Water absorption 32.2 ml/100 g BET surface area1.19 m²/g

EXAMPLE 6

The following components were mixed thoroughly:

220 g alpha alumina(II)** 290 g Hydrated alumina (Gibssite) 60 gBoehmite 15 g K15M Methocel (methyl cellulose) 50 g Polycarbonate powder25 g Mineral oil 20 Talc 115 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1350° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier E. Testingthe carrier showed that it has the following specifications:

Crush strength 21.9 lb Water absorption 30.5 ml/100 g BET surface area1.19 m²/g

EXAMPLE 7

The following components were mixed thoroughly:

510 g Hydrated alumina (Gibssite) 195 g Boehmite 15 g K15M Methocel(methyl cellulose) 75 g Polycarbonate powder 25 g Mineral oil 8 g 50% Tisolution (as titanium bis-ammonium lactato dihydroxide) 20 g Talc 10 gammonium fluoride 195 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1250° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier F. Testingthe carrier showed that it has the following specifications:

Crush strength 20.5 lb Water absorption 73.7 ml/100 g BET surface area0.91 m²/g

EXAMPLE 8

The following components were mixed thoroughly:

389 g Hydrated alumina (Gibssite) 111 g Boehmite 18 g K15M Methocel(methyl cellulose) 5 g EXPANCEL 551 25.0 g mineral oil 7.8 g 50% Tisolution (as titanium bis-ammonium lactato dihydroxide) 20 g Talc 108 gWater

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1375° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier G. Testingthe carrier showed that it has the following specifications:

Crush strength 14.8 lb Water absorption 33.0 ml/100 g BET surface area1.27 m²/g

EXAMPLE 9

The following components were mixed thoroughly:

60 g alpha alumina(I)* 60 g alpha alumina(II)** 280 g Hydrated alumina(Gibssite) 80 g Boehmite 28 g K15M Methocel (methyl cellulose) 5 gExpancel 551 25 g mineral oil 7.8 g 50% Ti solution (as titaniumbis-ammonium lactato dihydroxide) 20 g Talc 108 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1325° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. This carrier is designated as carrier H. Testingthe carrier showed that it has the following specifications:

Crush strength 23.8 lb Water absorption 31.2 ml/100 g BET surface area1.28 m²/g

EXAMPLE 10

The following components were mixed thoroughly:

170 g alpha alumina(II)** 333 g Hydrated alumina (Gibssite) 167 gBoehmite 15 g K15M Methocel (methyl cellulose) 75 g Polycarbonate powder25.0 g mineral oil 8 g 50% Ti solution (as titanium bis-ammonium lactatodihydroxide) 20 g Clay (hydrous aluminum silicate) 120 g Water

Mixing, shaping, and drying these components followed the same procedureas in Example 1. The firing scheme involved temperature ramping at arate of 4° C./min. up to 1375° C. The temperature of the furnace wasthen held at this level for 2 hours before it was allowed to cool downat a rate of 5° C./min. Testing the carrier showed that it has thefollowing specifications:

Crush strength 12.7 lb Water absorption 58.0 ml/100 g BET surface area0.72 m²/g

EXAMPLE 11 a. Preparation of a Stock Solution of Silver/Amine Complex:

A silver solution was prepared using the following components (parts areby weight):

Silver oxide 834 parts Oxalic acid 442 parts De-ionized water 2808 partsEthylenediamine 415 parts

Silver oxide was mixed with water, at room temperature, followed by thegradual addition of the oxalic acid. The mixture was stirred for 15minutes and at that point, the color of the black suspension of silveroxide had changed to the light brown color of silver oxalate. Themixture was filtered and the solids were washed with 3 liters ofde-ionized water.

The sample was placed in an ice bath and stirred while ethylenediamineand water (as a 66%/34% mixture) were added slowly in order to maintainthe reaction temperature lower than 33° C. After the addition of all theethylenediamine/water mixture, the solution was filtered at roomtemperature. The clear filtrate was utilized as a silver/amine stocksolution for the catalyst preparation.

b. Promoters Addition:

The clear stock solution was diluted with the 66/34 mixture ofethylenediamine/water. In addition, Cs hydroxide and ammonium hydrogensulfate were added to the diluted silver solution in order to prepare acatalyst containing 11% silver, and the suitable amount of cesium andsulfur.

c. Catalyst Impregnation:

A 150 g sample of the carrier was placed in a pressure vessel and thenexposed to vacuum until the pressure was reduced to 50 mm Hg. 200 ml ofthe adjusted silver/promoters solution was introduced to the flask whileit is still under vacuum. The pressure of the vessel was allowed to riseto atmospheric pressure and its contents were shaken for few minutes.The catalyst was separated from the solution and was now ready forcalcination.

d. Catalyst Calcination:

Calcination, deposition of silver, was induced by heating the catalystup to the decomposition temperature of the silver salt. This wasachieved via heating in a furnace that has several heating zones in acontrolled atmosphere. The catalyst was loaded on a moving belt thatentered the furnace at ambient temperature. The temperature wasgradually increased as the catalyst passed from one zone to the next. Itwas increased, up to 400° C., as the catalyst passed through sevenheating zones. After the heating zones, the belt passed through acooling zone that gradually cooled the catalyst to a temperature lowerthan 100° C. The total residence time in the furnace was 22 minutes. Theatmosphere of the furnace was controlled through the use of nitrogenflow in the different heating zones. This catalyst is designated ascatalyst 3.

c. Catalyst Testing:

The catalysts were tested in a stainless steel tube that was heated by amolten salt bath. A gas feed mixture containing 15% ethylene, 7% oxygen,and 78% inert, mainly nitrogen and carbon dioxide, was used to test thecatalyst at 300 p.s.i.g. The temperature of the reaction was adjusted inorder to obtain a standard ethylene oxide productivity of 160 Kg perhour per m³ of catalyst.

EXAMPLE 12

Carriers A-H were used to prepare catalysts for the oxidation ofethylene to ethylene oxide in a procedure illustrated in Example 11. Theresults of the catalyst testing are summarized in Table 1.

TABLE 1 Results of catalyst testing Reaction Catalyst CarrierSelectivity Temp ° C. 12-a A 83.7 237 12-b B 82.9 227 12-c C 84.2 23912-d D 82.9 227 12-e E 82.2 231 12-f F 81.9 244 12-g G 84.2 230 12-h H84.3 232

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove and all equivalents thereto.

What is claimed is:
 1. A composition for producing an alpha aluminacatalyst support, the composition comprising at least one alpha aluminahaving a median particle size of at least 5 microns, in an amount ofabout 20 percent to about 60 percent by weight of total alumina in thecomposition; at least one hydrated precursor of alpha alumina in anamount of about 40 percent to about 80 percent by weight of totalalumina in the composition; and a binder, wherein the composition issubstantially free of seeding particles.
 2. The composition of claim 1,wherein the composition is free of seeding particles.
 3. The compositionof claim 1, wherein the at least one hydrated precursor of alpha aluminacomprises an aluminum hydroxide.
 4. The composition of claim 3, whereinthe aluminum hydroxide is selected from the group consisting ofgibbsite, boehmite, diaspore, bayerite, and combinations thereof.
 5. Thecomposition of claim 1, wherein the composition is substantially free offerric oxide, chromium oxide, and sub-micron size particles of alphaalumina.
 6. The composition of claim 1, further comprising water in anamount sufficient to render the composition extrudable.
 7. Thecomposition of claim 1, wherein the binder comprises a material selectedfrom the group consisting of thermally decomposable organic compounds,clays, silica, silicates of elements of Group II of the Periodic Tableof the elements, and combinations thereof.
 8. The composition of claim1, wherein the binder comprises a material selected from the groupconsisting of polyolefin oxides, oil, acacia, carbonaceous materials,cellulose, substituted celluloses, cellulose ether, stearates, waxes,granulated polyolefins, polystyrene, polycarbonate, sawdust, ground nutshell flour, silica, an alkali metal salt, and combinations thereof. 9.The composition of claim 1, wherein the composition further comprisestalc.
 10. The composition of claim 1, wherein the composition furthercomprises a water-soluble titanium compound.
 11. The composition ofclaim 1, wherein the composition further comprises a burnout material.12. The composition of claim 1, wherein the composition furthercomprises a blowing agent.
 13. The composition of claim 12, wherein theblowing agent propels a gas upon the application of sufficient heat. 14.The composition of claim 1, wherein the composition further comprisesboron.
 15. A process for producing an alpha alumina catalyst support,the process comprising: a) preparing a composition comprising at leastone alpha alumina having a median particle size of at least 5 microns,in an amount of about 20 percent to about 60 percent by weight of totalalumina in the composition; at least one hydrated precursor of alphaalumina in an amount of about 40 percent to about 80 percent by weightof total alumina in the composition; and a binder, wherein thecomposition is substantially free of seeding particles; b) molding thecomposition into a structure; c) heating said structure for a sufficienttime and at a sufficient temperature to form a porous structure; and d)calcining the porous structure at a temperature above 1000° C. for asufficient time to convert the at least one hydrated precursor of alphaalumina to a porous and fused alpha alumina structure.
 16. The processof claim 15, wherein step c), step d), or both step c) and step d), areconducted in an atmosphere of an inert gas.
 17. A process for producinga catalyst, the process comprising producing an alpha alumina catalystsupport according to claim 15, and then e) depositing a catalyticallyeffective amount of silver onto a surface of said catalyst support. 18.The process of claim 17, further comprising depositing a promotingamount of a promoter onto the surface of the catalyst support, thepromoter comprising one or more alkali metal-containing compounds, oneor more transition metal-containing compounds, one or more sulfurcomponents, one or more fluorine-containing components, or a combinationthereof.
 19. A process for the oxidation of ethylene to ethylene oxide,the process comprising vapor phase oxidation of ethylene with molecularoxygen in a fixed bed tubular reactor in the presence of the catalystproduced by the process of claim
 17. 20. The catalyst produced by theprocess of claim
 18. 21. The composition of claim 1, wherein said atleast one hydrated precursor of alpha alumina is in an amount of about75 percent to about 80 percent by weight of total alumina in thecomposition.